The present invention relates to an adaptor for matching the uniform contact raster array in an apparatus for electronically testing printed circuit boards to the possibly non-uniform pattern of connection points on a printed circuit board to be tested.
A "univeral adaptor" as disclosed in EP-B1-26 824 comprises a platen overlying a basic raster array of a test apparatus and having there through passages or through-bores arranged in accordance with the aforesaid uniform contact raster array. At some distance above the basic raster array on the side of the platen facing away therefrom, there is provided an assembly of two adaptor plates held in a spaced relationship by means of stationary spacer elements. These adaptor plates are provided identically with bores arranged in accordance with the connection points on the circuit board to be tested. The bores in the adaptor plates receive needle-shaped and longitudinally resilient test pins the lower portions of which are resiliently deflectable in directions transverse to the longitudinal extent thereof to compensate for alignment errors between the aforesaid bores in the adaptor plates and passages in the platen. The adaptor also includes means for aligning the circuit board to be tested with the adaptor, such as suitable alignment pins engaging corresponding alignment bores in the circuit board under test. The two (inner and outer) adaptor plates have identical patterns of bores therethrough in accordance with the connection points on the circuit board to be tested and are held in a spaced relationship so that the portions of the test pins adjacent the printed circuit board are precisely vertical and perpendicular to the circuit board surface and accurately contact the connection points thereon even though the lower, more distant portions of the test pins may be deflected from vertical.
Complex circuit boards may have thousands of connection points thereon and a separate resilient test pin is needed for every one of them. As a consequence, expenses for an adaptor of this kind will rise disproportionately with the number of connection points as longitudinally resilient test points require a substantial manufacturing effort. In addition, test pins of this type have a minimum diameter which it is practically impossible to reduce further, so that there exists a natural limit to the test pin density which may be attained in a testing adaptor of the kind at issue.
In order to make possible the use of longitudinally rigid test pins which are easy to manufacture and particularly thin, there has been created an "active basic contact array" in which the contact elements of the basic raster array in the circuit board testing apparatus are substantially mounted for resilient movement in the direction of the longitudinal axis of the test pins. This active basic raster array is placed on the rigid hard-wired basic contact array in the printed circuit board testing apparatus and serves to make sure that the rigid test pins exert a contact pressure as uniform as possible and of sufficient magnitude on all the connection points they contact.
EP-A1-0 215 146 discloses an adaptor of this type ("Adaptor 85") including a top plate which has a pattern of bores therethrough in accordance with the circuit board to be tested and which engages the circuit board under test and wherein rigid uncontoured test pins extend through an additional plate made of an elastomeric material which prevents the pins from dropping out while the adaptor is being handled outside the testing apparatus proper. This makes it possible to save the substantial work required for providing an adaptor with its complement of test pins when testing a circuit board identical with one which has been tested some time in the past, since the adaptor may be shelved for re-use. The aforesaid adaptor is advantageous in that it makes possible the use of totally uncontoured rigid test pins. Providing it with its complement of test pins is made difficult, however, by the test pins being unable to simply "drop" into it, since some force must be applied to urge them through the elastomeric plate.
The adaptors discussed above were conceived for the one-side testing of printed circuit boards. A type of circuit board which is finding increasing use has connection points on both its major surfaces, however, so that they must be tested from both sides as well. It would be possible, of course, to test both sides individually one after the other. However, it would not be possible this way to detect conductor discontinuities as may occur in conductive connections between both sides of a circuit board, which connections moreover are particularly susceptible to opening. For this reason, the industry has developed circuit board testing apparatus capable of simultaneously testing both sides of double-sided printed circuit boards. This has lead to the necessity of providing adaptors for testing apparatus of this nature. Unfortunately, the adaptors developed in the past are suitable for simultaneous testing on both sides in exceptional cases only, for a non-uniform distribution of the connection points on the two sides of the circuit board may cause the many thousands of test pins to substantially warp and deform the board under test, which may cause additional or new faults in conductors or may even cause the circuit board to break, the latter particularly in the case of ceramic boards. Where a circuit board is tested on one side only, a non-uniform distribution or local concentrations of test pins cannot produce so harmful an effect since the supportive element engaging the opposite side of the circuit board under test may be a solid plate in the testing apparatus proper which will receive whatever forces are applied thereto non-uniformly without warping so that the circuit board under test will be supported in a condition substantially free from deformation. When a circuit board is tested on both sides at the same time, this manner of supporting it will not be practicable because there will exist no backing element on the respective other side of the circuit board under test in localized areas thereof where no test pins exert counter-pressure. At most, such backing element will have the form of a perforate adaptor plate of the other adaptor, which cannot be provided with any degree of rigidity, particularly since it often is made of acrylic or the like to facilitate the manual mounting of the test pins. In the past, therefore, the adaptors for the two sides of a double-sided circuit board testing machine were different, thus resulting in considerable additional expenditures.